This invention relates to an improvement in magnetic fluid, and more definitely it relates to an improvement on hue and electrical properties of magnetic fluid.
Usually, magnetic fluid is a liquid in which fine magnetic particles, such as fine magnetite particles having a particle size of 50-200 .ANG., are suspended in a dispersing medium by the aid of a surfactant. It has a dark brown color and keeps stable for a long time to stay substantially free from sedimentation of flocculation. As a dispersing medium for such magnetic fluid, there is available paraffin oil, ester oil, silicone oil, water and the like, and as a surfactant, there may be used carboxylic acids such as oleic acid, linoleic acid, etc., as well as cationic surfactants and nonionic surfactants.
Magnetic fluid finds a wide variety of uses in many fields of industry, including use as a sealing agent, lubricant, sink-float separation medium, oil-water separating agent, recording material and so on.
The magnetic fluid of this invention is usable in these fields of commercial products, and in particular it exhibits excellent properties in application to the field of recording material.
As image recording techniques utilizing a magnetic fluid, there are known the ink jet method and the magnetofluidographic use (hereinafter referred to as MFG method) disclosed in Japanese Patent Kokai (Laid-Open) No. 23534/79. The present invention is intended to provide a magnetic fluid which is particularly useful for the MFG method in which the magnetic fluid is let fly under the action of an electric or magnetic force to thereby form an image.
As magnetic fluid used for MFG method, there are available those magnetic fluids which use paraffin, ester, ether or the like as a base oil with their concentration being suitably adjusted, or those further including a colorant such as dye or pigment. Magnetic fluid containing no colorant forms an image having a light brown or dark brown color hue corresponding to the magnetic particles in the fluid, and the image quality is degraded due to such brown color hue. For changing the hue of the magnetic fluid from light brown or dark brown to black, it is conceivable to disperse a dye or a pigment in the magnetic fluid, but what comes into question here is separation of the colorant and magnetic particles and possible decomposition, aggregation and/or segregation of the colorant particles in the recording section of the apparatus. In the MFG method, in order to let the ink fly, a meniscus of the magnetic fluid is formed on a fine stylus (about 60 .mu.m in diameter) by a magnetic force, and a pulse voltage of several hundred volts is given to the stylus in correspondence to the image signal. Thus, the magnetic fluid is placed in an intense electric field (several thousand V/mm). Therefore, in use of the magnetic fluid as a magnetic ink for recording, if no consideration is given to the electrical properties of the dye or pigment added as colorant, there may arise a situation where the colorant particles get caught on the stylus or undergo aggregation or segregation with the result that the obtained image presents a color different from that of the original ink or assumes the very color of the magnetic particles carrying no colorant, resulting in a poor image quality.